As DRAMs increase in memory cell density, there is a continuing challenge to maintain sufficiently high storage capacitance despite decreasing cell area. Additionally, there is a continuing goal to further decrease cell area. One principal way of increasing cell capacitance is through cell structure techniques. Such techniques include three-dimensional cell capacitors, such as trenched or stacked capacitors. Yet as feature size continues to become smaller and smaller, development of improved materials for cell dielectrics as well as the cell structure are important. The feature size of 256 Mb DRAMs and beyond will be on the order of 0.25 micron or less, and conventional dielectrics such as SiO2 and Si3N4 might not be suitable because of small dielectric constants.
Highly integrated memory devices, such as 256 Mbit DRAMs and beyond, are expected to require a very thin dielectric film for the 3-dimensional capacitor of cylindrically stacked or trench structures. To meet this requirement, the capacitor dielectric film thickness will be below 2.5 nm of SiO2 equivalent thickness.
Insulating inorganic metal oxide materials (such as ferroelectric materials, perovskite materials and pentoxides) are commonly referred to as “high K” materials due to their high dielectric constants, which make them attractive as dielectric materials in capacitors, for example for high density DRAMs and non-volatile memories. In the context of this document, “high K” means a material having a dielectric constant of at least 10. Such materials include tantalum pentoxide, barium strontium titanate, strontium titanate, barium titanate, lead zirconium titanate and strontium bismuth titanate. Using such materials might enable the creation of much smaller and simpler capacitor structures for a given stored charge requirement, enabling the packing density dictated by future circuit design.
Despite the advantages of high dielectric constants and low leakage, insulating inorganic metal oxide materials suffer from many drawbacks. For example, all of these materials incorporate oxygen or are otherwise exposed to oxygen for densification to produce the desired capacitor dielectric layer. Densification or other exposure to an oxygen containing environment is utilized to fill oxygen vacancies which develop in the material during its formation. For example when depositing barium strontium titanate, the material as-deposited can have missing oxygen atoms that may deform its crystalline structure and yield poor dielectric properties. To overcome this drawback, for example, the material is typically subjected to a high temperature anneal in the presence of an oxygen ambient. The anneal drives any carbon present out of the layer and advantageously injects additional oxygen into the layer such that the layer uniformly approaches a stoichiometry of five oxygen atoms for every two tantalum atoms. The oxygen anneal is commonly conducted at a temperature of from about 400° C. to about 1000° C. utilizing one or more of O3, N2O and O2. The oxygen containing gas is typically flowed through a reactor at a rate of from about 0.5 slm to about 10 slm.
Certain high K dielectric materials have better current leakage characteristics in capacitors than other high K dielectric materials. In some materials, aspects of a high K material which might be modified or tailored to achieve a highest capacitor dielectric constant possible but will unfortunately also tend to hurt the leakage characteristics (i.e., increase current leakage). One method of decreasing leakage while maximizing capacitance is to increase the thickness of the dielectric region in the capacitor. Unfortunately, this is not always desirable.